Friday, November 03, 2006

A few pics to end the week...

Well, after all the furore caused by Miksang, this week has been the quietest for a while. Hope the "lively debate" didn't scare anyone off...

Anyhoo, to kick-start things again, I have posted a couple of images to get some discussion going. I'm not even going to hope that these can be seen large-ed up - I have no clue why my posts don't enlarge on clicking, but if anyone else wishes to try an experient, I'll email you the same file and you can post it to see what happens...until then, enjoy in minature.

Sunset in Kejimikujik (Ked-gee-ma-koo-jick) National Park, NS, Canada


This was taken April 2005, while Kate and I were on holiday with Damian and Sue. We spent a couple of days canoeing around the lake and islands, and spent 2 nights camping on our own private island.

It was bloody freezing, although mostly sunny. We had a bit of rain on the 2nd night and found that our tent was not waterproof. Damian and Sue gave us theirs, to allow us soft "City Slickers" to remain dry and comfortable. Going to the loo in the night was like something out of "The Blair Witch Project", but we all survived. I had a bit of a "moment" while we crossed from the mainland to the relative shelter of one of the islands. See, where I'm from (Britain), we can usually see over to the other side of most lakes. This one was frickin huge, and had a sea-like swell on it. V scary. Almost didn't go over, but there was a slight lull in the swell, and we all dashed over, Kate with Sue and me with Damo, for "safety" reasons. Glad we did - we had a great time and I was able to capture this beautiful sunset on the first evening.



St. Margaret's Headland, Kent, UK

And now for something totally different. Grey skies and a photographic challenge of a different sort.

Taken on my birthday, around 6.30 am, February 2004, this image of St. Margaret's Headland in Kent tries to bring out the essence of Britain's coast in the Winter months.

I tried to compose to use the curve of the rocks to lead into the chalk of the cliffside, and was lucky that there was a relatively interesting sky behind. The low light, and my desire for a good DoF, meant that I had to use a tripod to get a sharp image. The resultant 2 second exposure has just nicely blurred the water in the foreground, giving an unplanned, but very nice, effect to the otherwise uninteresting grey sea. The white rock may be a little distracting, taking the eye somewhat away from the interesting rock and seaweed detail on the foreground rocks. This was pre D-SLR days, and my Nikon Coolpix 5400 compact - as is the way with compacts - intrinsically has a greater DoF at a given aperture, so it was easier to get front-to- back sharpness. I think this is a result of the distance from lens to sensor meaning that f8 on a compact is more like f22 on a D-SLR, but the DoF is greater...I read it somewhere, can't remember exactly.

Anyway, comments/questions welcomed as ever.

Have a good weekend all.
Cheers
Ivan

5 comments:

Gareth Marlow said...

Nice shots both, although I think I prefer the second. It does look bloody freezing!

The DOF thing with compacts is because they have smaller sensors -> for a given angle of coverage, they need a shorter focal length lens than for a DSLR (in the same way as a DSLR needs a shorter focal length for a given angle of coverage than a 35mm camera) -> for a given aperture, you get more DOF, the shorter the focal length.

Ivan said...

Yes - the smaller sensor means that, for example, 10mm on a very small compact may be the same as 50mm on 35mm film scale, due to the sensor being 5x smaller than a 35mm film cell. However, regarding the aperture issue, this is not the reason why f8 on a compact is roughly the same as f22 on a DSLR. I thought that the aperture issue was due to the distance between the lens elements and the sensor being shorter in small compacts. Thus, the light has less distance to "spread out" beyond the cross over point of absolute focus, therefore has a smaller "circle of confusion" (the measure of the smallest resolvable point = sharpness), and therefore f8 appears like f22 (small aperture, small coc) on a camera where sensor and lens glass are further apart. Therefore, compacts have an apparent greater intrinsic DoF (or rather a more limited range of f-stops, usually 2.8 to 11) and this means they are more forgiving (i.e.focal point positioning less important) in terms of general focussing, hence their utility as point and click for beginners..phew! I think that's the reason for the aperture thing (lens to sensor distance) with sensor size being responsible for the focal length multiplier, which also has a positive impact on DoF.

Gareth Marlow said...

Hi Ivan,

Oh man, I've been racking my brains on this one since you posted, trying to work out if what you've said is right! I think the missing factor is the enlargement of the subsequent image. It's the size of the circle of confusion when enlarged which is important - not on the negative/sensor. If you're ending up with an 8x10 image with a fixed starting field of view, you have to enlarge a dcc sensor many more times than you'd enlarge a 35mm neg and you wouldn't have to enlarge a sheet of 8x10 film at all.

It took me ages to get my head around this, but there's a well-structured analysis here:

http://www.normankoren.com/Tutorials/MTF6.html

Come about 2/3 down the page to "DOF limits, diffraction, and format" and he re-arranges the equation for DOF. The distance from the lens to the film plane is not a variable in the equation.

My head hurts.

Ivan said...

I'm gonna make it hurt some more then..... ;-) !!!

I see what you are saying, and I think we are in some agreement, but what I was trying to get at is that the CoC is defined by the set up of the lens/sensor, since the distance from lens element to sensor MUST impact the physics of how the light rays move in relation to the focal point and the sensor plane, and therefore MUST define the initial size of the CoC in the first place. If the scaling up arguement was right, then the opposite to what is observed would be true: that is, smaller sensors would have a larger CoC in relation to larger sensors/film, and hence would have a shallower DoF, and this is not the observation...now mine hurts!

Gareth Marlow said...

Hi Ivan,

I'm not sure that the last bit holds. The CoC is governed by the distance to the sensor plan, the aperture, the focal length and the distance to the subject - we definitely agree on that. Because of the increased enlargement, the acceptable on-sensor CoC size is actually smaller for a small-sensor than a large one - I think that follows too. But the dominant factor is the rate at which the circle of confusion size changes as the subject gets closer to the lens - the cone of confusion, if you will (bloody hell!). The shape of that cone - the rate of change of size of the circle of confusion, is a function ONLY of the aperture and the focal length.

Splat. My head's finally gone. Anyway, what I think we both absolutely agree on is the super DOF achieveable from small sensor cameras, whatever the optical physics behind it is. And what we can also agree on is that that beach shot is super :)

It was such a shame that you couldn't make Camera Club tonight, because the photography from the guy who gave the talk was pant-wettingly good. He shot all of his landscapes on a 100-400mm lens and I think it's the sort of work you'd really love. Amazing Namibian sand dunes and his work from Yellowstone was out of this world - forest shots in this beautiful golden September light.

In the second half he talked about his portraiture - environmental travel portraits from India, shot mainly at about 20mm using (I'm guessing) the Canon 17-40 f4L, mainly at f4. Absolutely incredible.